An ejector cooling system with thermal pumping effect (ECSTPE) consumes no electricity but suffers severe thermal energy waste problems. This study innovatively reveals that different from the conventional ejector cooling where the designed coefficient of performance (COP) increases with an increasing set generating temperature, the designed COP of ECSTPE would increase first and then decrease with an increasing set generating temperature. To improve the designed performance of ECSTPE, an optimal set generating temperature is proposed, which corresponds to the peak designed COP and can be quantified using the formulated designed COP. Existence mechanisms and performance improvements to the optimal set generating temperature are theoretically analyzed and verified to be universal using case studies, particularly for an ECSTPE with environmentfriendly refrigerants. The results showed that ECSTPEs with more severe thermal energy waste problems exhibited greater performance improvements while using the optimal set generating temperature. At the set evaporating temperature of 10 °C and set condensing temperature of 30 °C for ECSTPEs using R1234yf, R290, R134a, R161, and R1234ze(E), the optimal set generating temperatures were 78.4 °C, 80.6 °C, 82.7 °C, 84.1 °C, and 86.2 °C, respectively. Setting the generating temperature higher than the optimal value (the conventional method) caused designed COP deteriorations of 6.51%/°C, 5.73%/°C, 2.83%/°C, 2.26%/°C, and 0.89%/°C, respectively. Performance improvements were greater for a higher set evaporating temperature and lower set condensing temperature; the latter had a greater effect. Therefore, the use of an optimal set generating temperature can effectively improve the designed performance of an ECSTPE.